Most air conditioning systems require a control system for controlling the air conditioning load, especially during peak outdoor temperature periods. A thermostat provides a control means for controlling an air conditioning system to maintain a temperature in an indoor space that is comfortable for the occupants. Energy considerations become important when the outside temperature substantially exceeds an indoor temperature set point. Most thermostat controls permit the user to set a desired set point temperature above which the air conditioner will energize to cool the space. However, when the outdoor temperature greatly exceeds the indoor temperature set point, a substantial energy load is required to maintain the indoor temperature. As a result, energy is wasted since the air conditioner maintains an indoor temperature that is lower than necessary to keep the occupants comfortable relative to the peak outdoor temperatures during the summer.
Prior control systems have attempted to control the indoor temperature based upon the outdoor temperature. U.S. Pat. No. 4,289,272 discloses a temperature control apparatus that operates by calculating a desired indoor temperature using a linear function of outdoor temperature. U.S. Pat. No. 4,089,462 discloses a temperature control system for shifting the indoor temperature set point with respect to outdoor temperature. U.S. Pat. No. 5,293,755 interrupts operation of an air conditioner when an allowable difference between indoor and outdoor temperatures is exceeded. While the control systems described in the above references provide means for air conditioning load reduction during peak periods, the above systems all require the installation and use of an outdoor sensor to measure the outdoor temperature.
It would be desirable, therefore, to provide a more simple control means for reducing the load of an air conditioning system during peak daytime temperature periods, which does not require the cost associated with the use or installation of an outside temperature sensor.
The present invention provides for a digital thermostat that comprises a software sub-routine for controlling the operation of an air conditioner and a stored value representative of the air conditioner""s operating time, which is indicative of the outdoor temperature and load on the air conditioning system. When the outdoor temperature greatly exceeds the indoor temperature set point, the load on the air conditioner increases, i.e., the air conditioner is on for greater periods of time and off for shorter periods of time. The stored value representative of operating time is evaluated by the software sub-routine, which responsively raises the set-point temperature when the stored value exceeds a predetermined level indicative of a high outdoor temperature. The raised set point results in a reduced load (on-time) for the air conditioner.
It is a further object of the invention to provide a digital thermostat that comprises a selectable xe2x80x9ceconomyxe2x80x9d mode of operation for enabling the above-described means for reducing the load on the air conditioning system. The user may select or deselect the economy mode as desired to suit the individual economic and comfort preferences of the user. The economy mode can be manually selected by the user, or remotely selected by the user or an Electric Power Utility provider to reduce energy consumption. An Electric Utility company may remotely access the thermostat via phone or Internet connection to select the economy mode as a way of shedding energy load demands during peak power usage periods.